J/30 Class Association Forums J/30 Forum The Boat, Sails and Equipment Transducers that fit original DataMarine mounting tube?

Dan - are you reusing the old housings (tubes)? If so, you probably can't do that due to different inside diameters. You can buy the flush mount Airmar housing from Defender. The housing should be properly bedded with 5200 so it won't leak.

In the interim, you might try the oversized o-rings or as an alternate build up the ID of the housing with a thin layer of plastic sheet like used for laminating paper. If you do this waterborne, you will need to blank off the hole from underneath.